This invention generally relates to an apparatus and method for inspecting the profile of the inner wall of a conduit, and is specifically concerned with inspecting the inner diameter profiles of Inconel.RTM. heat exchanger tubes in nuclear steam generators.
Probes for inspecting the inner walls of metallic conduits are known in the prior art. Such probes are particularly useful in inspecting the internal walls on the Inconel.RTM. tubes used as heat exchangers in nuclear steam generators for flaws or deformation caused by corrosion, fretting, or the accumulation of sludge products in the crevice regions of the generator. Generally, these probes operate by means of either strain gauges, or eddy current probes Strain gauge-type profilometer probes are generally formed from a cylindrical mandrel that is circumscribed by a plurality of the spring-loaded fingers. Strain gauges are placed onto each of the spring-loaded fingers. When the probe body is inserted into the interior of a tube and translated along its longitudinal axis, differences in the radius of the internal tube walls cause one or more of the spring fingers to flex in a radial direction. The extent to which these fingers flex is picked up by the strain gauges attached thereto. Eddy current-type profilometer probes are generally formed by an eddy current coil resiliently mounted in a probe head so as to wipingly engage the interior of the tube being inspected when the probe is rotated. The coil is electrically connected to a current generator which conducts an alternating current to the coil as it is moved. An impedance detecting circuit which may take the form of an inductive bridge is also connected across the leads of the coil. In operation, the alternating current conducted through the coil excites it into generating a pulsating magnetic field whose magnitude and polarity changes in accordance with the frequency of the current. When the coil of the probe is positioned in the vicinity of an electrically conductive wall, the changing magnetic flux emanating from the coil induces eddy currents in a portion of the wall. The particular amperage, voltage and direction of the eddy currents produced are dependent in part upon the specific impedance of the portion of the wall that conducts the eddy current. Because of the direction of flow of the eddy currents generated by the coil is opposite to the current flowing through the probe sensing coil, the magnetic field created by the eddy currents creates an impedance in the sensing coil. The strength of these eddy currents is in turn dependent upon the resistance that these currents encounter as they circulate through the wall. Since flaws in the metal wall (such as cracks, pits or regions of local thinning) create regions of higher resistances at flaw locations, eddy current probes may be used to locate flaws by constantly , monitoring the impedances of the sensing coils as the probe body is moved along the internal walls of the tube.
While some prior art profilometry probes are capable of performing satisfactory inspections of heat exchanger tubes, the applicants have noted a number of problems associated with these probes which has limited their usefulness.
Strain gauge-type profilometry probes tend to be delicate since they require the mounting of very small strain gauges onto the resilient metal fingers that circumscribe the probe body. The stain gauges themselves are small and delicate, as are their lead wires (which are hairline fine). Both the strain gauges themselves and their lead wires are prone to breakage if the probe is subjected to inadvertent mechanical shock, or is even rapidly drawn through an unusually rough portion of tube. While strain gauge-type profilometers are capable of detecting the presence of ovality in such tubes (which in turn indicates if the tube has been stressed as a result of intense, localized pressure, the flaw resolution of many of these types of profilometers is relatively coarse. If the flaw resolution is increased by the addition of more spring fingers and strain gauges around the circumference of the probe, the gauges must be made even smaller, which increases the fragility of the device even more.
Eddy current type profilometry probes can also suffer from excessive fragility in designs where a tiny coil resiliently engages the interior of a wall in wiping contact. While some of the better probe designs overcome this defect by either potting the eddy current probe in a self-lubricating plastic (which is subject to wear), or by attaching the coil to the back of a stylus which resiliently engages the inner tube wall as the probe is translated therein, none of these designs, to the knowledge of the applicants, is capable of accurately resolving tube ovality. Such ovality may occur when sludge deposits precipitate in the small annular space between the outer walls of the heat exchanger tubes and the bores of the support plates in the steam generators. The accumulation of such sludge deposits sometimes subjects one side or the other of the outer walls of the tubes to pressures great enough to render the cross section of the tube into an oval, and may even dent the tube. Such tube ovality is an important indicator of the potential for the tube to undergo stress corrosion cracking, which could lead to the undesirable result of the contamination of the nonradioactive water that is used to generate turbine-turning steam in the plant with the radioactive water which flows through the core of the nuclear reactor.
Clearly, there is a need for a profilometry probe that is sturdy enough in construction to reliably operate even when subject to inadvertent mechanical shock, but yet capable of accurately detecting tube ovality as well as other types of tube flaw such as pitting, cracking and wall thinning. Ideally, such a probe should be able to detect these flaws with even more accuracy and resolution than prior art eddy current type profilometry probes. Finally, it would be desirable if such a profilometry probe were easily manufactured from inexpensive and easily obtainable components.